The durability and service performance of an asphalt roadway can be affected by a number of variables, including the asphalt mix composition that is used, the construction techniques employed in building the roadway, the overall weight and the axle weight of the vehicles using the roadway, the number and speed of the vehicles and the temperature and other environmental factors under which the roadway is used. Most of these various factors are beyond the control of the road designer. Furthermore, as traffic has increased on the nation's highways and as high-pressure radial tires have become more commonly used on heavy trucks, wear and even deterioration of the roadways has accelerated. Therefore, construction techniques and especially the design and selection of asphalt paving mix compositions have assumed much greater significance than was previously the case. In particular, the design and selection of asphalt paving mix compositions have become more critical in building roads than ever before.
This increased importance in the design and selection of asphalt paving compositions has led to greater emphasis in recent years in developing testing apparatus and methods that can be useful in predicting the performance characteristics of asphalt paving compositions. The ability to predict such performance characteristics as rutting, stripping and fatigue cracking from a sample of a particular asphalt paving composition can lead to the development of improved and longer-lasting paving compositions.
Generally, pavement rutting will be due to inadequacy of the pavement system under the prevailing traffic and environmental conditions. Rutting is a manifestation of differential surface deformation in the wheel paths of a roadway which results from selective densification and shear deformation. Generally, the amount of pavement rutting depends, at least in part, on the traffic loads placed on the roadway and the distribution of such loads across the roadway. It will also be affected by the stresses introduced into the pavement system, and by the permanent strains induced as a result of these stresses. These permanent strains will depend on the permanent deformation characteristics of the layer materials, particularly the asphalt concrete layer.
Rutting reduces road serviceability and driving comfort and will reduce the service life of a roadway. In addition, rutting may also contribute to safety hazards that may arise from an accumulation of water in the rutting paths. Such accumulation may lead to hydroplaning, or in appropriate weather conditions, icing.
Because a tendency towards rutting can depend to a significant degree on the composition of the asphalt mix that is used, methods and apparatus for use in predicting rutting tendencies in various asphalt compositions have assumed increasing importance and emphasis. Beginning in 1985, the Georgia Department of Transportation and the Georgia Institute of Technology, in cooperation with the Federal Highway Administration and the U.S. Department of Transportation, embarked on a program to develop a testing machine that could accurately predict the rutting characteristics of highway paving compositions. This program resulted in the development of the Georgia Loaded Wheel Tester, an apparatus designed to test samples of asphalt compositions for rutting characteristics, and in the establishment of the GDT-115 testing procedure for rutting susceptibility. The study began with evaluation of a loaded wheel tester that had been used by the Georgia Department of Transportation Materials Testing Laboratory for design and testing of slurry seals. This machine was developed by Benedict Slurry Seal, Inc. of Dayton Ohio. It consists essentially of a one inch (2.54 cm) wide by three inch (7.62 cm) diameter hard rubber caster-type wheel mounted on a frame. A box to hold lead shot was mounted atop the wheel for providing a load. This loaded wheel was driven across a pavement sample through a twelve inch (30.48 cm) reciprocating stroke by a 0.25 hp (186.5 watt), 1750 rpm motor operated through a gear reducer at a ratio of 40:1 to provide 44 cycles per minute, where one cycle comprises two passes over the sample. The machine was operated in an airtight temperature-controlled room that was heated to simulate the effects of hot weather on rutting. The use of this apparatus did not provide satisfactory results, principally because it could not simulate the effect of high-pressure tires on the roadway. Subsequently, the Benedict apparatus was modified to include consideration of tire pressure on rutting. This modified machine, known as the Georgia Loaded Wheel Tester, employed an eight inch (20.32 cm) diameter aluminum wheel with a one inch (2.54 cm) diameter high-pressure rubber hose wrapped around it in the shape of a tire. The hose selected was capable of being pressurized at a pressure of up to 120 psi (828 kPa). This version of the apparatus was also found to be unsatisfactory, primarily because of excessive wear of the "hose-tire" due to skidding at the ends of the reciprocating stroke. This skidding also caused excessive rutting in the asphalt concrete samples at these locations.
The Georgia Loaded Wheel Tester was then modified to eliminate the "hose-tire" in favor of a high-pressure linearly disposed hose, which was placed on top of the asphalt concrete specimen and pressurized to simulate tire pressure. The hose was held in position on both ends by end clamps. A three inch (7.62 cm) diameter aluminum wheel with a concave rim was then attached to the reciprocating arm so as to ride atop the linear hose. As the machine was operated, the wheel riding along the hose was judged capable of simulating pressurized tire contact with the sample. The power rating of the motor for providing reciprocating motion was also increased to one-third horsepower (248.7 watts). The entire machine was also enclosed in an environmental chamber (instead of an environmentally-controlled room) which was capable of maintaining a constant temperature of up to 120.degree. F. throughout the test period. Enclosing only the test portion of the machine in an environmental chamber was considered but not deemed to provide sufficient temperature control as was required for test repeatability. This version of the Georgia Loaded Wheel Tester was also equipped with a preheating box to precondition pavement samples to the testing temperature prior to testing. In addition, a channel section having seven slots at two-inch (5.08 cm) intervals was developed to be placed over the sample for measuring the rut profile developed as a result of the testing. A dial gauge was used to measure the rutting developed at each of the seven locations in the sample defined by the slots in the channel section.
Finally, the machine was modified yet again. In the latest version of the Georgia Loaded Wheel Tester, the sample holder has been modified to accommodate three samples, and three loaded wheels are provided for simultaneous testing of the samples. Furthermore, in order to accommodate this expansion of the capacity of the machine, the portion of the apparatus undergoing reciprocating motion was changed from the loaded wheel or wheels to the sample or samples. The loaded wheels are held stationary, and the samples, each with its associated linearly disposed high-pressure hose, are moved back and forth therebeneath.
The Georgia Loaded Wheel Tester represents a significant advance in the technology of predicting rutting in pavement samples. However, it suffers from several shortcomings. Because it must be totally enclosed in an environmental chamber, it is not sufficiently portable to be readily capable of use in field laboratories. In addition, enclosing the entire machine in a chamber that is heated to simulate the effects of hot summertime weather puts the reciprocating motor and other components of the machine at risk of overheating. The Georgia Loaded Wheel Tester is also subject to excessive vibration during operation. The sample holder or carriage with its three samples is quite massive, and considerable vibration is developed by its reciprocating motion. Finally, although rutting is an important component of pavement failure, it is not the only such component. Prediction of susceptibility of pavement designs to other types of failure could also be quite valuable.
It would be advantageous, therefore, if a testing machine could be developed that is more portable and more versatile than the Georgia Loaded Wheel Tester, and that avoids its disadvantages and inadequacies.